The LaTeX command for longmapsto (⟼) is \longmapsto.
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Write \longmapsto inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
Arrows are relations in LaTeX, so \longmapsto gets the same thick spacing as an equals sign. With unicode-math under XeLaTeX or LuaLaTeX, typing ⟼ in a formula gives the same symbol.
$A \longmapsto B$Open this example in the live preview: it renders as you type, with no compiler and no signup.
\downarrow\updownarrow\nearrow\searrow\swarrow\nwarrow\mapsto\hookrightarrow\hookleftarrow\twoheadrightarrow\twoheadleftarrow\rightharpoonup\rightharpoondown\leftrightharpoonsArrows in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \longmapsto in math mode; it needs no extra package. For example, $A \longmapsto B$.
None: \longmapsto is built into LaTeX, so no extra package is required.